Traditionally rock crushing equipment that is used to reduce the size of high strength rock types has been manufactured in one of two different categories. These ‘crushers’ are categorised as either compression crushers or impact crushers. These two categories utilise two distinctly different processes to crush rock. Compression crushing physically loads rock particles between two metal surfaces, closing the gap between these surfaces during a crushing cycle and developing forces high enough to crack the trapped rock into multiple fragments. Impact crushing creates crushing forces via high velocity impacts of either metal on rock, rock on metal or rock on rock. Each method has its advantages and disadvantages. Compression crushing has the advantage of positive size reduction where the product size created is smaller than the feed size in a predetermined ‘reduction ratio’ which can be altered according to the ‘setting’ of the crushing apparatus. However, the compression crushing process indiscriminately reduces the size of all feed material and tends to produce a flaky, elongated product, which is undesirable for many applications. On the other hand impact crushing tends to discriminately crush weaker rock more and produce a more cubical shaped product which enhances the average strength of the product and is otherwise very advantageous in many applications. However, impact crushing suffers from the drawback that the size of the product is more variable and is dramatically influenced by a range of parameters. It is possible in some impact crushing situations for rock particles to pass through a crushing apparatus and emerge essentially unchanged in size. A further disadvantage of impact crushing is the high proportion of undesirable fine material produced in some applications, reducing the average value of the product. To utilise the advantages of each crushing process they are often used in conjunction with each other, where a number of compression crushing apparatuses will be used to reduce the size of the material down to the general product size range and then an impact crushing apparatus is used for the final ‘shaping’ and other quality improvement of the product.
There are many configurations of apparatuses in each category. Compression crushing apparatuses generally fall into two sub-categories: Jaw crushers, where the crushing surfaces are two flat plates; usually one moving and one stationary, and cone (or gyratory) crushers which utilise the layout of a gyrating cone within a stationary conical shell. The choice of compression crusher type for a particular application generally depends on the desired throughput vs. the feed size. Jaw crushing tends to be used in applications with a larger feed size at low to moderate production rates. Cone and gyratory crushing tends to be used in higher throughput applications where the feed size is smaller. Often crushing plants are constructed utilising multistage size reduction where a jaw crushing apparatus performs the initial size reduction and then cone crushing apparatuses are used for the subsequent size reduction. Both compression crushing types are generally constructed to crush hard and/or abrasive rock and both find economic use in a wide variety of rock types. Design parameters of greatest importance in both types of compression crushing apparatuses are: The maximum feed opening, the angle of the crushing surfaces relative to each other (the ‘nip’ angle), the setting (output size), the throw (the opening and closing movement of the crushing surfaces), and the speed. The optimum operating speed for a particular type of crushing apparatus is essentially a function of the preceding parameters. The flow of material through the crushing chamber occurs under gravitational force and is stopped (or ‘arrested’) during each crushing cycle. After each compression the stationary trapped rock particles accelerate under gravitational force, gaining speed downwards through the crushing chamber, until they are arrested by the next compression. Thus excessive crusher speed, which increases the number of compression cycles that the rock experiences during transit through the apparatus, actually reduces the crushing capacity by arresting the rock particles more frequently and reducing their average transit speed. In this sense compression crushing apparatus throughput is thus limited by gravity.
Impact crushing apparatuses also generally fall into two sub-categories: those where the crushing impact is created by metal components hitting rock (or vice versa), and those where the crushing impact is essentially rock hitting rock (so called ‘autogenous’ crushing). The choice of which type of impact crushing apparatus is used depends largely on the properties of the rock to be crushed. In abrasive rock types the autogenous crushing process is used almost exclusively, due to the uneconomic wear rates of metal components when they are subjected to high velocity, high abrasion impacts. The standard form of the autogenous impact crushing apparatus is that of a horizontal rotor, rotating on a vertical shaft, into which the rock to be crushed falls. The rock is thrown outwards by the spinning rotor under ‘centrifugal’ force and emerges from ports in the rotor at high speed to impinge on a bed of other rock surrounding the rotor. Such a configuration is known as a vertical shaft impactor (or VSI). The important design parameters of an autogenous VSI are; the feed opening, the rotor size and the rotation speed. The combination of rotor size and rotation speed determines the rim (or ‘tip’) speed of the rotor which governs the maximum level of kinetic energy available to the rock as it leaves the rotor. It is this available kinetic energy which largely controls the degree of size reduction achieved by the apparatus, and its power consumption, which is the dominant cost component in its operation. The operation of an autogenous VSI will now be described in more detail.
Referring to FIG. 1: As rock passes through the rotor at radial velocity Vr it is subjected to two perpendicular forces; centrifugal force Fr and coriolis force Ft. Centrifugal force acts in the radial direction out from the centre of rotation. Coriolis force acts tangentially in the plane and direction of rotation. These forces are governed by the following equations:Fr=mass×(rotation speed)2×radiusFt=mass×rotation speed×Vr×2
Thus the centrifugal force on a rock particle increases as it travels through the rotor (increasing radius) which tends to correspondingly accelerate it (that is, increase Vr exponentially). The coriolis force is proportional to Vr so as it speeds up the rock particle is subjected to more force from the surface it is travelling over. In a frictionless situation the rock would exit the rotor with Vr=Vt, (the tangential tip speed) and the coriolis force would be a maximum at the tip (the trailing edge of the port). The particle would exit the rotor at a relative angle of 45 degrees and its kinetic energy would be maximised, maximising the crushing forces available in its subsequent impact with the surrounding rock bed. In this situation the output kinetic energy of the rock particles would be exactly equal to the input rotational energy at the shaft. To describe this situation simplistically; the energy input at the shaft creates output kinetic energy that is 50% radial and 50% tangential. In a ‘real world’ situation where friction is involved the frictional drag created by the surface the rock is travelling over within the rotor provides a retarding force, reducing the rock's acceleration and consequently reducing the Vr it attains. In an autogenous VSI this surface is a bed of rock which builds up in the rotor, so designed to eliminate wear on the body of the rotor. Depending on the frictional characteristics of this rock bed the frictional force may limit Vr to a relatively low level as the feed rock exits the rotor. In this situation the coriolis force on the rotor tip at exit would be low, and the particles would exit the rotor more tangentially, but the kinetic energy of the exiting particle/s would be reduced. It is important to note however, that the input rotational energy at the shaft is the same as it would be in the frictionless situation. Thus, up to half the energy input at the shaft can be lost to internal friction within the rotor. This internal frictional loss provides no useful crushing action as the grinding action to which the rock particles are subjected to within the rotor only serves to create ultra-fine material, which is deleterious in most applications. Bearing in mind that autogenous VSI crushers are used primarily on abrasive rock types the designers of these crushers are forced to balance conflicting requirements: maximising Vr maximises kinetic energy output and thus overall energy efficiency, however it also increases both the coriolis force at the rotor tip and speed at which the rock particles ‘skid’ over the rotor tip. Thus the wear that the tip is subjected to increases dramatically with increasing Vr whereas minimising Vr decreases the tip wear but reduces the energy efficiency. Good rotor tip design is essential to control VSI operating costs and tips are made with ultra hard (tungsten carbide) inserts to give them an acceptable working life while maintaining relatively high Vr levels to improve energy efficiency. Patent No: NZ 168612 discloses the concept of an autogenous VSI while patents; NZ 201190, NZ 250027, NZ 274265, NZ 274266, NZ 299299, NZ 328061, NZ 328062 and NZ 502725 disclose various tip designs to enable rock bed creation within the rotor, with the effect being to limit Vr to acceptable levels. However, even with the benefit of these special tip designs autogenous VSI designers have been forced to limit input feed particle size dramatically to reduce coriolis force point loading and other tip impact loads.
It is an object of the present invention to address the foregoing problems or at least to provide the public with a useful choice.
All references, including any patents or patent applications cited in this specification are hereby incorporated by reference. No admission is made that any reference constitutes prior art. The discussion of the references states what their authors assert, and the applicants reserve the right to challenge the accuracy and pertinence of the cited documents. It will be clearly understood that, although a number of prior art publications are referred to herein; this reference does not constitute an admission that any of these documents form part of the common general knowledge in the art, in New Zealand or in any other country.
Throughout this specification, the word “comprise”, or variations thereof such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated element, integer or step, or group of elements integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.